1. Field of the Invention
The present invention relates to a servo write head assembly and servo writer that write a servo signal in a magnetic tape for performing tracking-control of a magnetic head.
2. Description of the Related Art
These years, in a magnetic tape a high density recording has progressed and there is the tape having a capacity of around 100 gigabytes for a backup of a computer. Therefore, several hundreds of data tracks are formed in the magnetic tape in a lateral direction. Accordingly, a width of a data track has extremely become narrow, and also a distance between adjacent data tracks has extremely become narrow. Therefore, in order to make a recording/reproducing element of a magnetic head trace the data track, a servo signal is written in the magnetic tape in advance, and with reading the servo signal by the magnetic head, a position thereof (position of the lateral direction of the magnetic tape) is servo-controlled (see Japanese Paten Laid-Open Publication No. Hei-8-30942).
And the servo signal is recorded on non magnetized magnetic tape so as to magnetize a servo band in one direction.
In other words, as shown in FIGS. 10A and 10B, conventional servo signals SS are formed on non magnetized servo bands SB by flowing a recording current pulse PC consisting of a zero current and a plus pulse current as a recording current in order to avoid a saturation phenomenon of a servo read element (MR (Magneto Resistive) element). If such the recording current pulse PC is used, a magnetic tape MT is not magnetized in areas except for servo patterns SP when the recording current pulse PC is the zero current; and when the plus pulse current of the recording current pulse PC flows, the servo patterns SP are magnetized in one direction by leak magnetic fluxes from servo gaps, thereby as a result the servo signals SS being written.
On the other hand, in a magnetic tape recoding/reproducing apparatus a change point of a magnetization in the servo signals SS is detected with a change of an electric resistance by an MR element, and the change point of the magnetization as a read signal is output in a differential waveform (voltage value). Therefore, the larger the electric resistance of the MR element becomes, the higher a peak voltage value of the read signal of the servo signals SS, thereby an SN (Signal/Noise) ratio of the read signal being improved. Accordingly, when changes of the servo signals SS themselves are large and a read area is large due to a wide width of the MR element, as shown in FIG. 10C a peak voltage value of a read signal RSL of the servo signals SS becomes high.
Whereas, hereafter the high density recording of the magnetic tape is foreseen to progress till around several tens of terabytes. Therefore, a number of data tracks of the magnetic tape, the width of the data track, and the distance between the adjacent data tracks will become narrower, and the magnetic tape itself will become a thin layer. Based upon this, an amount of magnetism that is detectable in reading a servo signal decreases and a change of a magnetization amount that is detectable with the servo read element also becomes small. Accordingly, as shown in FIG. 10D a peak voltage value of a read signal RSS of the servo signals SS becomes small, thereby the SN ratio of the read signal RSS worsening. As a result, in the magnetic tape recording/reproducing apparatus the servo signals SS becomes not able to be accurately read, thereby highly accurate position control of the magnetic head resulting in being not able to be performed.
Consequently, a servo write head assembly and servo writer, which can write a servo signal that is high in the SN ratio thereof, are strongly requested.